All Standard C++ header files must be included in the following way:

 #include  //note: no ".h" extension#include//...other #include statements

That is, the “.h” extension should not be used. This technique applies to a subset of standard C header files as well, with the addition of the letter ‘c’ affixed to their name. So a C standard header formerly named is now . For example:

 #include  //formerly:   mind the prefix 'c' and the omission of  ".h"

The older convention for C headers, , is still supported but is now considered deprecated and should not be used. The problem is that C headers would inject their declarations into the global namespace. In C++ most standard declarations are grouped under namespace std and so are the Standard C headers. This eliminates the name conflict problem which can occur when global declarations are used. Keep in mind that you need to use a using declaration or a using directive in order to access the declarations in the standard headers:

 #include  using namespace std; //this is a using directivevoid f(){	printf ("Hello World
");}